I’m trying to use Cloud Builder and it seems like the most up to date version of Xcode supported is 8.3.2.
Do you happen to have a secret sexy beta 9.x group that you could add me to? Happy to contribute errata to the cause.
I was hoping for this to work too 
My macbook is too old to support the latest XCode/MacOS
Following up to point out Xcode 9 support went out today, so ARKit should now be supported in Unity Cloud Build.